Our objective is to develop a procedure utilizing genetic complementation in intraspecific somatic cell hybrids to enable chromosomal mapping of a number of human genetic defects which cannot be mapped by current procedures. A Roberts Syndrome cell line with a cytogenetic anomaly correctable by genetic complementation will be studied in the development of this procedure. Roberts Syndrome is a rare autosomal recessive genetic defect which produces severe symmetrical limb reduction, exophthalmos, ocular hypertelorism and clitoral or penile enlargement. A majority of these patients also have an unusual chromosomal anomaly. This anomaly, termed the RS effect, is visible as a premature separation of the centromeres of some of the chromosomes and has been shown to be due to a general repulsion of C-band positive heterochromatic regions in sister chromatids. While this cytogenetic defect cannot be corrected by co-cultivation with normal human or murine cells, it has been corrected in our laboratory by fusing the Roberts Syndrome line to a murine cell line. This correction observed in interspecific somatic cell hybrids indicates that this defect can be corrected by the presence of a normal gene product. We will utilize this phenotypic correction as a tool in determining the chromosomal assignment of the Roberts Syndrome locus. To achieve this, normal human cells will be transformed with the plasmid pSV2-neo to provide a dominant selection system (Geneticin resistance) for the isolation of hybrid cells. Single chromosomes carrying the plasmid will be transferred to Roberts Syndrome cells via microcell fusion. The resulting hybrids will be analyzed cytogenetically to identify the transferred chromosome, which should appear as a trisomy, and to determine if the cytogenetic anomaly has been corrected. Through the analysis of a series of such hybrids, we will determine the chromosome which is consistantly associated with the correction of the anomaly. This approach should be especially useful in mapping any genetic disorder, whether it appears as a chromosomal, biochemical or morphological anomaly, which can be detected in cell culture but for which the abnormal gene product is unknown.